In modern high-speed aircraft, weight, space, and costs are highly important. Also, there is a heavy emphasis on the reliability and maintenance support aspects of the systems in these aircrafts since the reliability and maintenance impacts strongly on the availability and sortie rates of aircraft. Pursuant to this, a reduction of part counts and a reduction in complexity are key to the achievement of the high-reliability and low-maintenance criteria. Thus, the integration of the auxiliary power unit (APU), emergency power unit (EPU), environmental control system (ECS), and engine start system (ESS) with a correspondent reduction in the number of parts, weight and size is highly desirable. This is particularly true in that the APU is considered by the airlines and the military to be a low-utilization device that adds weight, complexion, and maintenance support to the airplane; also, it is considered as one that has little operational benefit during most flight conditions. For these reasons, the APU is not a welcome addition to the airplane; but, it is one that is essential thereto when independence of ground support power for engine starting, environmental control, and electrical power functions is necessary. Certainly in any advanced fighter-type aircraft which is subject to bare site dispersal and has to scramble rapidly, the APU system is inevitable.
The complexity of such elements of the secondary systems are readily apparent when viewing the prior art. Typical military aircraft use airframe mounted accessory drives (AMAD) mechanically coupled to the main engines for driving hydraulic pumps, electric generators, and other accessories. A separate EPU is then often provided which uses either a pressurized air bottle or mono/bi-propellant fuel to drive an air turbine motor inlet coupled to a generator and hydraulic pumps for emergency flight loads. The main APU itself is normally a ground-use unit which drives an electric generator, hydraulic pumps, and typically an air compressor to supply pressurized air for air conditioning and avionic cooling loads.
Some prior efforts have been made to integrate the functions of the APU and EPU to maximize the role of the various components thereof, as they have met some degree of success. An additional advantage offered by such integration is that a single-type energy power source such as aviation fuel could be used for the EPU as well as the APU. However, the shortcomings of these prior art systems are that there is still a good deal of component replication and the mechanical integration of the APU/EPU system was complex. Furthermore, the operational aspects of the systems in regard to engine starting and ground checkout are very protracted and inefficient.
Therefore, it is a primary object of the subject invention integrate the auxiliary power unit, the environmental control system, the emergency power unit, and the engine start system so as to improve reliability, reduce parts duplication, minimize maintenance support, and finally to reduce the size and weight of the aircraft.
It is a further object of the subject invention to integrate the above-mentioned systems so as to offset the low utilization factor problems of the conventional auxiliary power and emergency power units.